In the use of mobile telescopic cranes or mobile cranes with latticework booms, which in the present invention are collectively referred to as mobile cranes, there is frequently a need for supplying various devices on the main boom, or on an auxiliary boom attached to it, with hydraulic energy.
Usually, the hydraulic supply of all hydraulic loads arranged on a mobile crane is carried out using a single hydraulic pump arranged on the revolving superstructure and driven by a diesel engine also arranged on the revolving superstructure. With the aid of this very powerful hydraulic unit comprising the hydraulic pump and the diesel engine, all hydraulic loads arranged on the mobile crane are fed. Primarily, the hydraulic unit centrally arranged on the revolving superstructure serves to supply hydraulic energy to the pivoting apparatus also positioned on the revolving superstructure. Moreover, the telescopic sections extensible and retractable out of and into the base section are extended and retracted with the aid of the hydraulic unit centrally positioned on the revolving superstructure and with the aid of a hydraulically operable telescoping and locking apparatus arranged on the base section. For this purpose the locking and telescoping apparatus is linked with the hydraulic unit arranged on the revolving superstructure via hydraulic hoses which are wound on a hose drum.
Hydraulically pivotable main boom extensions are also operated with the hydraulic unit centrally arranged on the revolving superstructure. For this purpose, the pivoting cylinder necessary for pivoting the main boom extension up and down is centrally supplied with energy by the hydraulic unit arranged on the revolving superstructure via corresponding hydraulic hoses. In order to ensure secure guiding of the hydraulic hoses, the hoses are therefore wound on a driven or spring biased hose drum from which they are unwound during the telescoping action.
In addition, there is often a great number of additional hydraulic loads of the crane which are fed by the hydraulic unit. Principally, all hydraulic loads of the mobile crane are always fed through a star type hydraulic supply, wherein each individual hydraulic load is connected with the single hydraulic unit arranged on the revolving superstructure with an associated hydraulic line. This central hydraulic supply of the individual hydraulic loads is, however, very complex since each individual load is fed via separate hoses. This considerable technical complexity has had to be accepted, however, since solely the hydraulic unit on the revolving superstructure exists to supply the individual hydraulic loads.
As has been explained, this separate supply of the individual hydraulic loads is very complex and results in problems in particular with hydraulic loads at a great distance from the hydraulic unit due to very long hydraulic hoses. Since, due to the growing requirements on mobile cranes by the modern building industry, boom lengths are increasingly greater and longer, exceeding lengths of 100 meters, problems arise with the supply of piston-cylinder units, for example, those situated at the head of the main boom.
The supply of these hydraulic loads can no longer be economically or technically implemented with the arrangement described, since due to their great own weight as a consequence of the great main boom lengths the hydraulic hoses are so heavy that they tend to break at great heights. While the dimensions of the hoses and in particular their wall thicknesses and their diameters may be extended to a certain degree, this measure is limited by the maximum allowable winding volume.
Another problem arising with the supply of piston-cylinder units at a great height by means of a hydraulic unit centrally arranged on the revolving superstructure is that as a consequence of the hose length to be maintained wound up, for example, on the crane revolving superstructure, the hose is very heavy. The arrangement of very heavy components such as these very heavy hose drums is undesirable because of the maximum allowable axle load of 12 tons in Germany and because of the space needed by the large hose drum.
Indeed, it is known from EP 0 276 612 A1 or from JP 11286395 to provide a working platform being mounted to a telescopic arm with an autarchic hydraulic power unit which is adapted to maintain the working platform in its horizontal position by actuating hydraulic piston cylinder units. However, the arrangement of such a hydraulic power unit on a working platform which is always leveled in its horizontal position comprises the special characteristic, that the hydraulic power unit does only work accurately, when the working platform is really always leveled in its horizontal position. In the event of tilting of the working platform, the hydraulic power unit will tilt together with the working platform which in turn results already in the event of slight deflections from the horizontal position in that the hydraulic pump of the hydraulic power unit will run dry which will in turn result in a malfunction of the hydraulic power unit.